This invention relates to a resonant buck-type voltage converter which employs a swinging inductance to achieve zero-current switching.
A basic buck-type converter configuration is shown in FIG. 1A and is designated by the general reference character 100. The input source 122 is the primary supply with its positive terminal connected to node 102 and its negative terminal connected to the common ground reference node 120. Switch 104 is connected between node 102 and Phase node 106. The anode of diode 110 is connected to the common ground node 120 and its cathode is connected to Phase node 106. Output filter 108 is comprised of inductor 116 and capacitor 118. The output filter 108 input node is the inductor 116 first terminal and it also is connected to Phase node 106. The output filter 108 output node is connected to the buck converter 100 output terminal 112 and also to the first terminal of capacitor 118 and to the second terminal of inductor 116. The second terminal of capacitor 118 is connected to the common ground node 120. The load circuit 114 is connected between the output terminal 112 and the common ground node 120. Note that, for consistency and clarity, the same identification numbers are generally used throughout this description where the same elements are used in different embodiments.
The basic buck-type converter operates to transfer energy from the input source 122 to the output load 114 based on the state of switch 104. FIG. 1B shows the basic operation of the converter configuration of FIG. 1A with the voltage at the Phase node, VPhase, shown as waveform 130. When switch 104 is closed, i.e., in the on-state, VPhase is equal to Vin as energy is drawn from input source 122. When switch 104 is open, i.e., in the off-state, the current draw from the input source 122 is stopped, but the load 114 may continue to draw current from the Phase node 106 through the output filter 108 and the diode 110. This is shown in FIG. 1B as the high-to-low portion of waveform 130. The output voltage, Vout, is the average value, as indicated.
The switch 104 on-state time versus its off-state time is controlled by a Pulse Width Modulation (PWM) circuit that is not shown in FIG. 1A, but is common and well known in the art. The duty cycle of the PWM waveform, as shown and indicated in FIG. 1B, is close to 50%. This duty cycle is actually a function of the ratio of the output voltage, Vout, versus the input voltage, Vin. Where this ratio is relatively high, indicating that Vout is relatively close to Vin, the PWM waveform must have a relatively high duty cycle so that the switch 104 on-state time is greater than its off-state time. This allows more time for energy transfer from the input source. Where this ratio is relatively low, indicating that Vout is much less than Vin, the PWM waveform must have a relatively low duty cycle so that the switch 104 on-state time is much less than its off-state time. In this case, commutation losses can become significantxe2x80x94up to about 30% of the power flowing through the converter. Next generation microprocessors and highly integrated circuitry will operate at 1.3 volts or less and at currents of 45 amperes or more. In order to avoid having these extremely large currents on the backplanes of computer systems, primary power supply voltages will be larger than 1.3 volts, such as 5 volts or 12 volts, or possibly more in the future. Thus, the application trends are to supply lower output voltages from higher primary supplies, further exacerbating this potential commutation loss problem.
The commutation losses at such relatively high voltages lead to relatively large losses in electrical efficiency. Designers, at the same time, want to be able to operate the voltage converters at higher frequencies, e.g., 1 or 2 megahertz or more instead of 200 or 300 KHz as is presently the case. Increasing the number of commutations taking place within a given period of time like this will inevitably lead to higher losses. The desire, therefore, is to eliminate the commutation losses. This can be done by using resonant techniques to force the commutation to occur at a zero current point. With this realized, the efficiency can be raised to the 90% level, resulting in roughly ⅓ of the previous losses. Consequently, less power is thermally lost and the physical size of the voltage converter can therefore be made smaller. The voltage converters can then operate at higher frequencies without any thermal loss/efficiency penalty.
FIG. 2A shows a schematic diagram of a buck-type converter designated by the general reference character 150 and intended to take advantage of the resonant techniques discussed above. It is a modification of the circuit shown in FIG. 1A and the differences will be discussed herein. A resonant capacitor 156 is added with its first terminal connected to Phase node 106 and its second terminal connected to the common ground node 120. This capacitor provides a relatively soft dv/dt on Phase node 106. Switching element 126 is shown in its more common actual implementation as an n-channel MOS transistor device (NMOS). Any switching element could be used here, such as other types of transistors, including bipolar devices or isolated gate bipolar transistors (IGBTs). The switch 126 includes transistor 160 and parasitic diode 158. The transistor 160 source terminal is connected to node 152, its drain terminal is connected to node 102 and its gate terminal is connected to node 124. Node 124 receives the output of the PWM switch control circuit that is not shown, but is described above. In a series connection with transistor 160, the first terminal of inductor 154 is connected to node 152 and its second terminal is connected to Phase node 106. The inductance 154 may, with equal effect, be located on the other side of switch 126, i.e., between node 102 and switch 126. While the goal of this circuit is to switch at zero current to minimize the associated commutation losses, the current actually rings back to negative values and charge is sent back to the input source. The waveforms illustrating only one cycle of the cyclical operation of the circuit of FIG. 2A are shown in FIG. 3A. In FIG. 3A, waveform 240 shows the Iin current and the waveform is mapped through time point designations t0, tr, and t1 to the waveform 242 that shows the VPhase voltage. The switch-on time is from t0 through t1 and tr represents the resonant time point.
One way to block the negative part of the current waveform shown in FIG. 3A is to add a diode or a second transistor back-to-back with the switch transistor accepting the PWM signal of the circuit of FIG. 2A. FIG. 2B shows a schematic diagram of a buck-type converter designated by the general reference character 200 where block 210 effectively replaces switch 126 of FIG. 2A. In block 210, the switch 222 includes transistor 218 and parasitic diode 220. The transistor 218 source terminal is connected to node 202, its drain terminal is connected to node 214, which is also the positive terminal connection for input source 122, and its gate terminal is connected to node 212. Node 212 accepts the PWM signal for energy transfer control, as discussed above. The second element in block 210, sub-block 208, primarily performs a diode function to disallow negative current flow, but includes transistor 204 as well as parasitic diode 206. Of course, any diode, one-way switching element, or appropriate transistor, such as a bipolar transistor or IGBT, could be used here instead of the NMOS transistor including a parasitic diode, as shown. The transistor 204 source terminal is connected to node 202, its drain terminal is connected to node 216, and its gate terminal is connected to node 212 to also receive the PWM signal for energy transfer control. The waveforms illustrating only one cycle of the cyclical operation of the circuit of FIG. 2B are shown in FIG. 3B. In FIG. 3B, waveform 244 shows the Iin current and this waveform is mapped through time point designations t0, tr, and t1 to the waveform 246 that shows the VPhase voltage. The switch-on time is from to through t1 and tr represents the resonant time point. As is seen from the waveform 244, the current is not allowed to go negative because of the blocking diode action.
The problem with the circuit shown in FIG. 2B is that, at low input voltage, the diode or the second NMOS device of sub-block 208 reduces the conversion efficiency. The approximately 0.6V voltage drop across the diode limits the efficiency as the input voltage is reduced. The case as illustrated in FIG. 2B, where the NMOS transistor is used, is able to efficiently operate at lower Vin values than the corresponding single diode case, but the efficiency loss problem at low input voltage remains. So, for a circuit to yield high efficiency conversion over a relatively wide range of input and output voltage requirements, another solution is required.
In this invention, a swinging inductor is used to achieve zero-current switching in a resonant buck-type voltage converter. The swinging inductor exhibits the property of a relatively high inductance at zero current and low current, but it swings back to the original resonant inductance value at high current. So, the high and medium regions of the current sine wave remain effectively unchanged. However, upon circuit power application, the current rises slowly due to the high swinging inductance value, but this is an innocuous parasitic effect.
FIG. 4A shows the inductance versus current waveform 280 of a typical swinging inductance. There is commonly a 10X difference in the high inductance value as compared to the resonant inductance value. FIG. 4B shows two E-cores contained in the inductor construction where the gap between E-core 286 and E-core 288 affects the inductance curve of the manufactured swinging inductor. FIG. 4C shows two waveforms that illustrate the variations due to the gap between the E-cores. Waveform 290 exemplifies a small gap whereas waveform 292 exemplifies a large gap structure. The large gap structure inductor saturates more slowly and this is why curve 292 rolls off much slower as compared to curve 290. Other types of saturable inductors function as swinging inductors, such as those doped with non-magnetic materials which serve the physical function of the gap.
Using the swinging inductance placed in series with a switching device to control the transfer of energy from the input source to the output load allows for a very efficient transfer of energy. Moreover, the efficiency level is sustainable over a wide range of input and output voltage requirements.